From 1dfba10f79adc5fc3a79f307beb514e53945df6e Mon Sep 17 00:00:00 2001
From: milad <mza1989@gmail.com>
Date: Tue, 27 Dec 2016 12:39:19 +0330
Subject: [PATCH] finished exercises on Course.Extend

---
 README.markdown      |  2 +-
 src/Course/Extend.hs | 14 ++++++--------
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/README.markdown b/README.markdown
index b6b9f80..969bb8c 100644
--- a/README.markdown
+++ b/README.markdown
@@ -26,7 +26,7 @@ After this, the following progression of modules is recommended:
 * `Course.FileIO` -> Done
 * `Course.State` -> Done
 * `Course.StateT` -> Done
-* `Course.Extend`
+* `Course.Extend` -> Done
 * `Course.Comonad`
 * `Course.Compose`
 * `Course.Traversable`
diff --git a/src/Course/Extend.hs b/src/Course/Extend.hs
index 80295b8..edd7dff 100644
--- a/src/Course/Extend.hs
+++ b/src/Course/Extend.hs
@@ -33,8 +33,7 @@ instance Extend Id where
     (Id a -> b)
     -> Id a
     -> Id b
-  (<<=) =
-    error "todo: Course.Extend (<<=)#instance Id"
+  (<<=) f ida = Id $ f ida
 
 -- | Implement the @Extend@ instance for @List@.
 --
@@ -51,8 +50,8 @@ instance Extend List where
     (List a -> b)
     -> List a
     -> List b
-  (<<=) =
-    error "todo: Course.Extend (<<=)#instance List"
+  (<<=) _ Nil = Nil
+  (<<=) fla la@(_:.res) = fla la :. (<<=) fla res
 
 -- | Implement the @Extend@ instance for @Optional@.
 --
@@ -66,8 +65,8 @@ instance Extend Optional where
     (Optional a -> b)
     -> Optional a
     -> Optional b
-  (<<=) =
-    error "todo: Course.Extend (<<=)#instance Optional"
+  (<<=) _ Empty = Empty
+  (<<=) f a = Full (f a)
 
 -- | Duplicate the functor using extension.
 --
@@ -86,5 +85,4 @@ cojoin ::
   Extend f =>
   f a
   -> f (f a)
-cojoin =
-  error "todo: Course.Extend#cojoin"
+cojoin fa = id <<= fa